The practice of sharing syringes without adequate sterilization between successive users is a major contributor to the transfer of Human Immunodeficiency Virus (HIV) and Hepatitis with subsequent severe repercussions for the sufferer and at a high cost to society for supporting and providing medical attention to sufferers.
In response to this problem, syringes have been developed with the aim of preventing syringe re-use.
One solution has been to develop syringes where the needle is permanently retractable into the barrel of the syringe, retraction driven by a compressed spring, as for example described in International Publication WO 01/80930. An improved “feel” may be provided for a syringe user such as by incorporating a pre-compressed spring that does not provide resistance to plunger depression, as described in International Publication WO 2004/082747.
However, spring decompression is relatively uncontrolled, which in use can lead to excessively forceful needle retraction that can result in blood splattering as air is forced from the syringe barrel as the needle retracts into the barrel.
Another problem associated with retractable syringes is that the plunger must be alignable with the retractable needle so that complementary engaging means located on the plunger and retractable needle do not engage prior to retraction, but can engage when required to facilitate needle retraction.
This requirement for correct orientation can add to the manufacture cost and sale price of the retractable syringe, which can detract from the commercial attractiveness of the retractable syringe, particularly in relation to mass production and mass distribution in third world countries.